A critical need exists for elastomers capable of performing in extreme thermal environments. Silicone polymers represent a group of elastomers owing to their inherent thermal and oxidative stabilities. Silphenylene siloxane polymers are known to be stable at high temperatures. This is due in part to the presence of the rigid silphenylene moiety that interferes with the siloxane redistribution reaction. Silphenylene siloxane polymers have been synthesized and investigated by several research groups over the past several decades (see, for example, Dvornic, P. R. Lenz, R. W. High-Temperature Siloxane Elastomers; Huethig & Wepf Verlag: New York, 1990).
For example, Hundley and Patterson (N. H. Hundley and W. J. Patterson, “Formulation/Cure Technology for Ultra-High Molecular Weight Siphenylene-Siloxane Polymers” NASA Technical Paper 2476 (1985)) studied certain derivatives of silphenylene-siloxane (SPS) polymers having the formula shown below:
The main obstacle to use of these polymers and related carborane derivatives is their inability to be easily vulcanized to effect curing. Hundley and Patterson prepared derivatives of SPS polymers, wherein a vinyl group substituent replaced a methyl substituent, giving the modified SPS polymer formula shown below:
The inclusion of the vinyl substituent in such SPS polymer derivatives considerably improved curing by vulcanization. Importantly, such SPS polymer derivatives demonstrated improved thermal and oxidative stabilities over extant commercial silicone resin polymer formulations. Yet both elastomer formulations exhibited extensive degradation in mechanical properties after being exposed to 288° C. for 16 hr. (Id. at p. 10).
MacKnight and coworkers (U. Lauter et al. “Vinyl-Substituted Silphenylene Siloxane Copolymers: Novel High-Temperature Elastomers” Macromolecules 32, 3426-3431 (1999)) prepared and studied SPS polymer formulations that included 30˜70 percent vinyl substitution as depicted by one exemplary formula shown below:
While these derivatives displayed greater thermal stability than prior formulations, the high temperature limit for possible applications of these materials as fire-safe elastomers extends to about 230° C.
Homrighausen and Keller (C. L. Homrighausen and T. M. Keller, “High-Temperature Elastomers from Silarylene-Siloxane-Diacetylene Linear Polymers,” J. Polym. Sci. Part A: Polym. Chem. 40:88-94 (2002)) prepared and characterized linear silarylene-siloxane-diacetylene polymers having the formula shown below:
where n=1-3. Polymers that contain the vulcanizable acetylene moiety as part of the chain or as a pendant functional group are known in the art. In most cases, incorporation of the acetylene group improves the thermal stability of the respective polymers. The increase in thermal stability is believed to be due to generation of a cross-linked material. Yet elastomers based upon these polymers began to exhibit significant weight loss after a couple of hours at temperatures up to about 330° C. in air as determined by thermogravimetric analysis (TGA, Id.).
Additional compounds include those having phosphorous as a substituent, for example:
Most elastomeric polymers containing these species are also sensitive to thermal degradation. For example, the first structure in the table (CAS 1342156-21-1) was used in the preparation of polyester resins but their decomposition temperatures (5% weight loss T5%) are all below 300° C., rendering them ill-suited for long-term use at such temperatures.
Commercially available silicone-based elastomeric materials, such as that exemplified by room temperature vulcanized 60 (“RTV60”), lose their mechanical properties as they decompose at operating temperatures (for example, 316° C.) for a relatively short life span (for example, a few hundred hours). Thus, there is still a need for elastomeric materials having improved temperature stability, longevity and robust mechanical performance for prolonged periods of time at high temperatures.